The invention is related to a filter element for purifying a fluid at risk of freezing. The increasingly more stringent legal exhaust gas limits can be fulfilled by commercial vehicles according to the present state of the art only with the addition of special exhaust gas aftertreatment systems. SCR (selective catalytic reduction) catalysts are considered the most promising exhaust gas cleaning system at present; they greatly reduce the proportion of nitrous oxides of the exhaust gas.
For this purpose, by means of an injection valve provided in the exhaust gas system a 32.5% urea-water solution that is carried onboard the vehicle is metered into the exhaust gas. In the catalyst the urea is converted to ammonia so that the nitrous oxides contained in the exhaust gas are converted to harmless nitrogen and water.
In this technology, the injection nozzles for the urea-water solution as well as the other system components must be protected from wear by means of a filter. In this way, the service life of the metering unit is extended. The filter element is installed either on the pressure side or the suction side of the pump that pumps the urea solution from the storage container.
The freezing point of the aforementioned urea-water solution, without any flow-improving additives, is at −11 degrees Celsius. Accordingly, it cannot be excluded that the solution will freeze under certain climatic environmental conditions, in particular when the vehicle is parked. Since the solution will expand upon freezing, measures must be taken to prevent damage of the system components carrying the solution as a result of such expansion. The same problem also occurs for other fluids that are at risk of freezing and systems carrying such fluids.
EP 1 593 419 B1 discloses a liquid filter for fluids at risk of freezing whose filter body is of a closed ring shape and surrounds an annular chamber. In the annular chamber a support element is arranged which is slightly smaller than the annular chamber so that between the filter medium and the support element a minimal gap is present. The support element is largely impermeable for the fluid to be cleaned so that the volume that is taken up by the support element cannot be filled by the fluid. The minimal quantity of the fluid cannot cause a great volume expansion so that damage of the filter parts is thus prevented.
However, the fluid quantity contained in the annular chamber still has an existing expansion potential not to be neglected that acts onto the components of the filter and stresses them.